Cardiac ryanodine receptors (RyR2) play a central role in the process of excitation-contraction coupling in the heart. Alterations in RyR2 regulation are evident in a variety of cardiovascular diseases. Changes in the phosphorylation status of RyR2 have been described in patients with atrial fibrillation (AF), but the underlying abnormal mechanisms remain poorly understood. Considerable evidence suggests that RyR2 channels comprise a macromolecular complex with regulatory subunits including protein kinases and phosphatases. Studies in human atrial tissue and myocytes as well as experiments in knock-in mouse models have revealed that increased RyR2 phosphorylation predisposes to atrial ectopy and progression of AF. Our preliminary data suggest that alterations in the RyR2-bound protein phosphatases (PPs) might underlie elevated RyR2 phosphorylation in AF, although very little is currently known about this. The long-term goal of this project is to define the cellular/molecular mechanisms by which PPs regulate RyR2 activity in both normal hearts and in humans/mice with AF. The present proposal will test the general hypothesis that variation in levels of PP regulatory subunits associated with RyR2 contributes to enhanced sarcoplasmic reticulum (SR) Ca leak and vulnerability to AF. Specific aim (1) will use biochemical and genetic approaches to dissect the role of PPs in the regulation of RyR2 phosphorylation. Specific aim (2) will examine the effects of PP regulation of RyR2 on SR Ca handling in atrial cells using genetic mouse models. Specific aim (3) will determine the role of PP regulation of RyR2 in atrial fibrillation in vivo in mouse models. Significance: Abnormal RyR2-mediated SR Ca leak has been associated with increased propensity to AF, the most prevalent type of cardiac arrhythmia. The mechanisms will be studied at the molecular, cellular and in vivo level, using recombinant proteins, genetic mouse models and human atrial biopsies. It is anticipated that these multidisciplinary studies will provide fundamental and new insights into the molecular mechanisms by which the RyR2 calcium release channel becomes dysregulated in AF. These insights may guide future developments of anti-arrhythmic drugs for the treatment of AF.